How should funding agencies pay open-access fees?

" OA journals are currently at a significant disadvantage with respect to subscription journals, because universities and funding agencies subsidize the costs of subscription journals in such a way that authors do not need to trade off money used for the subsidy against money used for other purchases....The solution is clear: universities and funding agencies should underwrite reasonable OA publication fees just as they do subscription fees. But how should this be done? Each kind of institution needs to provide its fair share of support....Of course, many funders already allow grantees to pay for OA publication fees from their grants. But this method falls afoul of some of [my five] criteria....One of the nice properties of this approach is that it doesn’t require synchronization of the many actors involved. Each funding agency can unilaterally start providing OA fee reimbursement along these lines. Until a critical mass do so, the costs would be minimal. Once a critical mass is obtained, and journals feel confident enough that a sufficient proportion of their author pool will be covered by such a fund to switch to an open-access revenue model, subscription fees to libraries will drop, allowing for overhead rates to be reduced commensurately to cover the increasing underwriting costs...."